3GPP Long Term Evolution (3GPP LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology, such as UMTS (Universal Mobile Communications System), are currently deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE. The work item (WI) specification on LTE called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP TR 25.913, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN),” version 8.0.0, January 2009 (available at http://www.3gpp.org and incorporated herein by reference).
Component Carrier Structure in LTE (Release 8)
The downlink component carrier of a 3GPP LTE (Release 8) is subdivided in the time-frequency domain in so-called sub-frames. In 3GPP LTE (Release 8) each sub-frame is divided into two downlink slots as shown in FIG. 1, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each sub-frame consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each of OFDM symbol spans over the entire bandwidth of the component carrier. The sub-frames thus each consist of a number of 2·NsymbDL modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 2.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block” (or “physical resource block”, abbreviated PRB). A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 2. In praxis, the downlink resources are assigned in resource block pairs (or physical resource block (PRB) pairs). A resource block pair consists of two resource blocks on the same subcarriers of the sub-frame, i.e. spans NscRB consecutive subcarriers in the frequency domain and the entire 2·NsymbDL modulation symbols of the sub-frame in the time domain. NsymbDL may be either 6 or 7, so that a sub-frame has either 12 or 14 OFDM symbols in total.
In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.9.0, December 2009 section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
The value NRBDL depends on the downlink transmission bandwidth configured in the cell and shall fulfill the relation NRBmin,DL≦NRBDL≦NRBmax,DL. Presently, NRBmin,DL=6 and NRBmax,DL=110 and represent the smallest and largest downlink bandwidths, respectively, supported by the current version of the specifications in 3GPP LTE (Release 8) and 3GPP LTE-A (Release 10)—see below. NSCRB is the number of subcarriers within one resource block. For a normal cyclic prefix sub-frame structure, NSCRB=12 and NsymbDL=7 in the current versions of the 3GPP specifications for 3GPP LTE (Release 8) and 3GPP LTE-A (Release 10).
In MBSFN operation, the user equipment receives and combines synchronized signals from multiple cells. In order for MBSFN reception, the user equipment performs a separate channel estimation based on MBSFN Reference Signal (MBSFN RS). In order to avoid mixing MBSFN RS and other reference signals (RSs) in the same sub-frame, certain sub-frames, known as MBSFN sub-frames, are reserved for MBSFN transmission.
Up to two of the first OFDM symbols within a sub-frame are reserved for non-MBSFN transmission and the remaining OFDM symbols are used for MBSFN transmission. In the first up to two OFDM symbols, PDCCH for uplink resource assignments and PHICH can be transmitted, and the cell specific reference signal is the same as non-MBSFN sub-frames.
The pattern of MBSFN sub-frames in one cell is broadcasted in the System Information (SI) of the cell. UEs, that are not capable of receiving MBSFN, will decode the first up to two OFDM symbols and ignore the remaining OFDM symbols.
MBSFN sub-frame configuration supports both 10 ms and 40 ms periodicity. And sub-frames #0, #4, #5 and #9 cannot be configured as MBSFN sub-frames.
General Structure for Downlink Physical Channels
The general downlink 3GPP LTE (Release 8) baseband signal processing according to 3GPP TS 36.211 section 6.3 is exemplarily shown in FIG. 6. Further details on the LTE downlink can be found in 3GPP TS 36.211, section 6. A block of coded bits is first scrambled. Up to two code words can be transmitted in one sub-frame.
In general, scrambling of coded bits helps to ensure that receiver-side decoding can fully utilize the processing gain provided by channel code. For each codeword, by applying different scrambling sequence for neighboring cells, the interfering signals are randomized, ensuring full utilization of the processing gain provided by the channel code. The scrambled bits are transformed to a block of complex modulation symbols using the data modulator for each codeword. The set of modulation schemes supported by LTE downlink includes QPSK, 16-QAM and 64-QAM corresponding to 2, 4 or 6 bits per modulation symbol.
Layer mapping and precoding are related to MIMO applications. The complex-valued modulation symbols for each of the code words to be transmitted are mapped onto one or several layers. LTE supports up to four transmit antennas. The antenna mapping can be configured in different ways to provide multi antenna schemes including transmit diversity, beam forming, and spatial multiplexing. Further the resource block mapper maps the symbols to be transmitted on each antenna to the resource elements on the set of resource blocks assigned by the scheduler for transmission. The selection of resource blocks depends on the channel quality information.
Downlink control signaling is carried out by three physical channels:                PCFICH to indicate the number of OFDM symbols used for control signaling in a sub-frame (i.e. the size of the control channel region)        PHICH which carries downlink ACK/NACK associated with UL data transmission        PDCCH which carries downlink scheduling assignments and uplink scheduling assignments.        
Downlink Reception in 3GPP LTE
In 3GPP LTE (Release 8), where there is only once component carrier in uplink and downlink, the PCFICH is sent at a known position within the control signaling region of a downlink sub-frame using a known modulation and coding scheme. As the determination of the downlink resources assigned to the user equipment depends on the size of the control signaling region of the sub-frame, i.e. the number of OFDM symbols used for control signaling in the given sub-frame, the user equipments needs to decode the PCFICH in order to obtain the signaled PCFICH value, i.e. the actual number of OFDM symbols used for control signaling in the sub-frame.
If the user equipment is unable to decode the PCFICH or obtains an erroneous PCFICH value, this PCFICH detection error will result in the user equipment not being able to correctly decode the L1/L2 control signaling (PDCCHs) comprised in the control signaling region, so that all resource assignments contained therein are lost.
Physical Downlink Control Channel (PDCCH) and
Physical Downlink Shared Channel (PDSCH)
The physical downlink control channel (PDCCH) carries scheduling grants for allocating resources for downlink or uplink data transmission. Each scheduling grant is defined based on Control Channel Elements (CCEs). Each CCE corresponds to a set of Resource Elements (REs).
In 3GPP LTE, one CCE consists of 9 Resource Element Groups (REGs), where one REG consists of four consecutive REs (in the frequency domain) excluding potential REs of reference signals.
The PDCCH for the user equipments is transmitted on the first NsymbPDCCH OFDM symbols (either 1, 2 or 3 OFDM symbols as defined by the PCFICH) within a sub-frame. The region occupied by the NsymbPDCCH in the time domain and the NRBDL×NscRB subcarriers in the frequency domain is also referred to as PDCCH region or control channel region. The remaining NsymbPDSCH=2·Nsymb−NsymbPDCCH OFDM symbols in the time domain on the NRBDL×NscRB subcarriers in the frequency domain is referred to as the PDSCH region or shared channel region (see below).
For a downlink grant on the physical downlink shared channel (PDSCH), the PDCCH assigns a PDSCH resource for (user) data within the same sub-frame. The PDCCH control channel region within a sub-frame consists of a set of CCE where the total number of CCEs in the control region of sub-frame is distributed throughout time and frequency control resource. Multiple CCEs can be combined to effectively reduce the coding rate of the control channel. CCEs are combined in a predetermined manner using a tree structure to achieve different coding rate.
In 3GPP LTE, a PDCCH can aggregate 1, 2, 4 or 8 CCEs. The number of CCEs available for control channel assignment is a function of several factors, including carrier bandwidth, number of transmit antennas, number of OFDM symbols used for control and the CCE size, etc. Multiple PDCCHs can be transmitted in a sub-frame.
On a transport channel level, the information transmitted via the PDCCH is also refereed as L1/L2 control signaling. L1/L2 control signaling is transmitted in the downlink for each user equipment (UE). The control signaling is commonly multiplexed with the downlink (user) data in a sub-frame (assuming that the user allocation can change from sub-frame to sub-frame). Generally, it should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis where the TTI length (in the time domain) is equivalent to either one or multiple sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, then the L1/L2 control signaling needs only be transmitted once per TTI.
Furthermore, the PDCCH information sent on the L1/L2 control signaling may be separated into the Shared Control Information (SCI) and Dedicated Control Information (DCI).
The physical downlink shared channel (PDSCH) is mapped to the remaining OFDM symbols within one sub-frame that are not occupied by the PDCCH. The PDSCH resources are allocated to the user equipments in units of resource blocks for each sub-frame.
FIG. 8 shows the exemplary mapping of PDCCH and PDSCH within a normal sub-frame (having 2·Nsymb=14 OFDM symbols in the time domain), respectively a resource block pair (see magnification). The first NPsymbPDCCH=2 OFDM symbols (PDCCH region) are used for L1/L2 control signaling, i.e. for signaling the PDCCH, and the remaining NsymbPDSCH=12 OFDM symbols (PDSCH region) are used for data. Within the resource block pairs of all sub-frames, cell-specific reference signals, CRS (Common Reference Signal), are transmitted. These cell-specific reference signals are transmitted on one or several of antenna ports 0 to 3. In this example, the CRS are transmitted from two antenna ports: R0 is from antenna port 0 and R1 is from antenna port 1.
Furthermore, the sub-frame also contains UE specific reference signals, DM-RS (DeModulation Reference Signal) that are used by the user equipments for demodulating the PDSCH. The DM-RS are only transmitted within the resource blocks where the PDSCH for a certain user equipment is allocated. In order to support MIMO (Multiple Input Multiple Output) with DM-RS, four DM-RS layers are defined, which means at most MIMO of four layers is supported. In the example of FIG. 8, DM-RS layer 1, 2, 3, are 4 are corresponding to MIMO layer 1, 2, 3, and 4.
FIG. 9 shows another example where the PDCCH and the PDSCH is mapped to a MBSFN sub-frame. The example of FIG. 8 is quite similar to FIG. 8, except for the MBSFN sub-frame not comprising common reference signals.
For further information on the LTE physical channel structure in downlink and the PDSCH and PDCCH format, see St. Sesia et al., “LTE—The UMTS Long Term Evolution”, Wiley & Sons Ltd., ISBN 978-0-47069716-0, April 2009, sections 6 and 9. Additional information on the use of reference signals and channel estimation in 3GPP LTE can be found in section 8 of this book.
Further Advancements for LTE—LTE-Advanced (3GPP LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07) in November 2008. Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved which is also referred to as “Release 10”. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
In order to extend the overall system bandwidth, LTE-A (Release 10) uses carrier aggregation, where two or more component carriers as defined for LTE (Release 8)—see FIG. 1 and FIG. 2 discussed above—are aggregated in order to support wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation. It is commonly assumed that a single component carrier does not exceed a bandwidth of 20 MHz.
A terminal may simultaneously receive and/or transmit on one or multiple component carriers depending on its capabilities:                An LTE-Advanced (Release 10) compatible mobile terminal with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers. There is one Transport Block (in absence of spatial multiplexing) and one HARQ entity per component carrier.        An LTE (Release 8) compatible mobile terminal can receive and transmit on a single component carrier only, provided that the structure of the component carrier follows the Release 8 specifications.        
It is also envisioned to configure all component carriers LTE (Release 8)-compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are same. Consideration of non-backward-compatible configurations of LTE-A (Release 10) component carriers is not precluded. Accordingly, it will be possible to configure a user equipment to aggregate a different number of component carriers of possibly different bandwidths in the uplink and the downlink.
Relaying Functionality—
Introduction of Relay Nodes to the UTRAN Architecture of 3GPP LTE-A
Relaying is considered for LTE-A as a tool to improve e.g. the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas.
The relay node is wirelessly connected to radio-access network via a donor cell. Depending on the relaying strategy, a relay node may be part of the donor cell or may control cells of its own. In case the relay node is part of the donor cell, the relay node does not have a cell identity of its own (but may still have a relay ID). In the case the relay node is in control of cells of its own, the relay node controls one or several cells and a unique physical-layer cell identity is provided in each of the cells controlled by the relay.
At least “Type 1” relay nodes will be part of 3GPP LTE-A. A “type 1” relay node is a relaying node characterized by the following:                The relay node controls cells, each of which appears to a user equipment as a separate cell distinct from the donor cell.        The cells should have its own Physical Cell ID (defined in 3GPP LTE (Release 8)) and the relay node shall transmit its own synchronization channels, reference symbols, etc.        In the context of single-cell operation, the user equipment should receive scheduling information and HARQ feedback directly from the relay node and send its control channels (SR/CQI/ACK) to the relay node        The relay node should appear as a 3GPP LTE-compliant eNodeB to 3GPP LTE-compliant user equipments (i.e. be backwards compatible)        To 3GPP LTE-A-compliant user equipment, a “type 1” relay node should appear differently than 3GPP LTE-compliant eNodeB to allow for further performance enhancement.        
An exemplary network structure using relay nodes in 3GPP LTE-A is shown in FIG. 3. The link between donor eNodeB (d-eNB) and relay node is also referred to as relay backhaul link. The link between relay node and user equipments attached to the relay node (r-UEs) is called relay access link.
Propagation Delay between Node B and Relay Node
In the following, a network configuration as shown in FIG. 3 is assumed for exemplary purposes. The donor eNode B transmits L1/L2 control and data to a so-called macro-user equipment (UE1) and also to a relay (relay node), and the relay node transmits L1/L2 control and data to a so-called relay-user equipment (UE2).
Further assuming that the relay node operates in a time-duplexing mode, i.e. transmission and reception operation are not performed at the same time, we arrive at a non-exhaustive entity behavior over time as shown in FIG. 4. Whenever the relay node is in “transmit” mode, UE2 needs to receive the L1/L2 control channel and physical downlink shared channel (PDSCH), while when the relay node is in “receive” mode, i.e. it is receiving L1/L2 control channel and PDSCH from the Node B, it cannot transmit to UE2 and therefore UE2 cannot receive any information from the relay node in such a sub-frame.
The situation becomes somewhat trickier in case that the UE2 is not aware that it is attached to a relay node. As will be understood by those skilled in the art, in a communication system without relay node any user equipment can always assume that at least the L1/L2 control signal is present in every sub-frame.
In order to support such a user equipment in operation beneath a relay node, the relay node should therefore pretend such an expected behavior in all sub-frames. This leads to a behavior as shown in FIG. 5. The relay node has to transmit the L1/L2 control channel in each sub-frame (here assumed to be in the early part of each sub-frame), before it can switch to reception mode. Additionally shown is a “Gap” which is required to tune the relay node hardware and software from “transmit” to “receive” mode and vice versa, which is typically a fraction of a sub-frame. What can be seen is that effectively the time that is available for transmission from a Node B to a relay node is actually only a fraction of a sub-frame, as indicated in the figure by the dashed box. In 3GPP Release 8, the UE2 behavior shown for sub-frame 2, i.e. to receive only the first part identical to the L1/L2 control signaling, can be achieved by configuring that sub-frame as an “MBSFN sub-frame”. Since this is done mainly to tell the UE2 to not process or expect the remainder of that sub-frame, it is also sometimes called a “fake MBSFN sub-frame”. In LTE, a node transmitting such “fake MBSFN” sub-frames is required to transmit the first two OFDM symbols of such a sub-frame before it can switch to reception.
As shown in FIG. 6, it can be usually assumed that more than a single relay node is deployed and connected to a Node B. In addition, it is possible that the relay node is not stationary, but can be mobile as a user equipment. For example, a relay node can be installed in a public transportation vehicle such as a bus, train, or tramway. In any case, the distance between Node B and at least one relay node is variable, so that different propagation delay for the signal from Node B to relay nodes will occur.
Using the exemplary deployment of FIG. 6, FIG. 7 illustrates the situation assuming that the relay nodes' transmission is synchronized to the Node B's transmission, as it is for example beneficial for the case that a user equipment should easily hand over between the Node B and a relay node or for simultaneous multipoint transmission purposes. For the first two OFDM symbols of the fake MBSFN sub-frame, Node B, RN1, and RN2 transmit simultaneously. Then for the relay nodes the first gap is required to switch to reception mode, followed by reception of the Node B transmission signal until just before the end of the sub-frame, where the second gap is required by the relay nodes to switch back again to transmission mode before the beginning of the next sub-frame.
As can be seen, depending on the length of the gaps and propagation delay for the signal between Node B and RN1 and between Node B and RN2, a relay node will be able to see only a limited and at least partially different set of OFDM symbols transmitted by the Node B. For RN1, the reception of OFDM symbol #1 overlaps with the gap, as does the reception of OFDM symbol #12. For RN2, the reception of OFDM symbol #2 overlaps with the gap, as does the reception of OFDM symbol #13. While RN1 can see OFDM symbols #2 to #11 completely, RN2 can see OFDM symbols #3 to #12 completely. Assuming a simple and cost-effective receiver at the relay node, partially invisible OFDM symbols cannot be used since they would contain a lot of interference and should therefore be considered as corrupt.
It may be thus assumed that the relay node is not able to detect the early part of a sub-frame transmitted by a Node B, which usually carries L1/L2 control information. Therefore, the Node B of transmitting to the relay node may use only those OFDM symbols within a R-PDCCH region within a sub-frame for conveying the L1/L2 control information to the relay node that can be received by the relay node.
Relay Backhaul Sub Frames
If the eNodeB-to-relay node link operates in the same frequency spectrum as the relay node-to-UE link, simultaneous eNodeB-to-relay node and relay node-to-UE transmissions on the same frequency resource may not be feasible due to the relay transmitter causing interference to its own receiver, unless sufficient isolation of the outgoing and incoming signals is provided. Therefore, when relay node transmits to donor eNodeB (d-eNB), it cannot receive signals from the user equipments attached to the relay node (r-UEs). Likewise, when relay node receives from donor eNodeB (d-eNB), it cannot transmit to user equipments attached to the relay (r-UEs), as has been explained above with respect to FIG. 4 and FIG. 5.
Thus, there is sub-frame partitioning between relay backhaul link (eNodeB-to-relay node link) and relay access link (relay node-to-UE link). Currently it has been agreed that:                Relay backhaul downlink sub-frames, during which eNodeB to relay node downlink backhaul transmission may occur, are semi-statically assigned.        Relay backhaul uplink sub-frames, during which relay node to eNodeB uplink backhaul transmission may occur, are semi-statically assigned or implicitly derived by HARQ timing from relay backhaul downlink sub-frames.        
In relay backhaul downlink sub-frames, the relay node will transmit to the donor eNodeB and r-UEs are not supposed to expect any relay transmission. In order to support backward compatibility for r-UEs, the relay node configures the backhaul downlink sub-frames as MBSFN sub-frame. As shown in FIG. 5, the relay backhaul downlink sub-frame consists of two parts. In the first OFDM symbols (up to two), the relay node transmits to r-UEs as it would do for a normal MBSFN sub-frame. In the remaining part of the sub-frame, relay receives from donor eNodeB, so there is no relay node to r-UE transmission in this part of the sub-frame. r-UEs receive the first OFDM symbols (up to two) and ignore the rest part of the sub-frame.
MBSFN sub-frame can be configured for every 10 ms and 40 ms. Hence, relay backhaul downlink sub-frames also support both 10 ms and 40 ms configuration. Also similar to the MBSFN sub-frame configuration, relay backhaul downlink sub-frames cannot be configured at sub-frames #0, #4, #5 and #9. Those sub-frames that are not allowed to be configured as backhaul downlink sub-frames are called “illegal downlink sub-frames” here.
Relay downlink backhaul sub-frames can be normal sub-frames (as exemplified in FIG. 8) or MBSFN sub-frames (as exemplified in FIG. 9).
Relay Backhaul R-PDCCH Region
As outlined with respect to FIG. 5 and FIG. 7 above, the relay node cannot receive L1/L2 control information (PDCCH) from donor eNodeB within the first OFDM symbols of the sub-frame.
Thus, a new physical control channel (R-PDCCH) is used to dynamically or “semi-persistently” assign resources within the semi-statically assigned sub-frames to the relay node for the downlink and uplink backhaul data. The R-PDDCH(s) for the relay node is/are mapped to a R-PDCCH region within the PDSCH region of the sub-frame. The relay node expects to receive R-PDCCHs within this region of the sub-frame. In time domain, the R-PDCCH region spans the configured downlink backhaul sub-frames. In frequency domain, the R-PDCCH region exists on certain resource blocks that are configured for the relay node by higher layer signaling.
R-PDCCH has following characteristics:                Within the physical resource blocks (PRBs) semi-statically assigned for R-PDCCH transmission, a subset of the resources is used for each R-PDCCH. The actual overall set of resources used for R-PDCCH transmission within the above mentioned semi-statically assigned PRBs may vary dynamically between sub-frames.        These resources may correspond to the full set of OFDM symbols available for the backhaul link or be constrained to a subset of these OFDM symbols.        The resources that are not used for R-PDCCH within the above mentioned semi-statically assigned PRBs may be used to carry R-PDSCH or PDSCH.        The R-PDCCH is transmitted starting from an OFDM symbol within the sub-frame that is late enough so that the relay can receive it.        Both frequency distributed and frequency localized R-PDCCH placement are supported.        Interleaving of R-PDCCHs within limited number of PRBs can have diversity gain and at the same time limit the number of PRBs that could be wasted.        In normal sub-frames, 3GPP LTE-A DM-RS (DeModulation Reference Signal) is used when DM-RS are configured by eNodeB, otherwise 3GPP LTE CRS (Common Reference Signal) is used.        In MBSFN sub-frames, 3GPP LTE-A DM-RS is used.        
The mapping of the R-PDCCH control information to the R-PDCCH region within the PDSCH region of the backhaul downlink sub-frames is one of the topics discussed in 3GPP RANI working group.